The present invention relates to an exposure control device for a photocopier which automatically controls the exposure in an electronic photocopier, for example.
With an electronic photocopier of this type, especially in the case of manuscripts with dark backgrounds such as newspapers and dark blueprints, it is necessary to control to increase the exposure or increase the bias voltage of the developer as compared with general manuscripts. However, such control has conventionally been performed by the operator as the need arises by, for example, operating an exposure adjusting dial or a change-over switch mounted on the control panel. However, such control was defective in that the operation is complex, the operability is inferior, the optimal exposure for various kinds of manuscripts is difficult to determine, and suitable photocopying cannot be constantly performed. A prior art device is disclosed in U.S. Pat. No. 3,926,518 of Berry et al.
This device is an optical scanning system with means for detecting illuminance adjacent to the optical path. The illuminance is adjusted in response to an output signal from the illuminance detecting means. However, this invention does not teach to vary the brightness of the exposure lamp. Furthermore, a particular circuit for the illuminance adjusting means is not disclosed.
In U.S. Pat. No. 3,743,405 of Morse et al, an exposure control device is disclosed which consists of a radiation-sensitive control element. However, this invention does not teach to control the amount of light of the exposure lamp by comparing a reference signal with a voltage sum of a voltage corresponding to a voltage applied to the exposure lamp and a voltage corresponding to the reflected light from the manuscript, unlike in the case of the present invention.